There are a wide variety of piston configurations in current use. A common type is one that employs several packings at each end of the piston with a retainer made of Teflon, or hard rubber, along with backup rings. Other piston configurations employ O-rings and U-rings in tandem with backup rings, while other pistons have split-type rings and quad-type seals made of rubber. On heavier equipment, such as automobile engines, split cast iron piston rings are used which require a tool to collapse the rings for entry into the cylinder. The use of seals, O-rings and packings add considerable overall cost to the piston construction.
It has also been proposed to use ringless pistons for certain applications. In pistons of this type, a rubber or resilient outer section is either chemically bonded or mechanically interlocked with a metal core, and the outer resilient section is provided with lips which, under fluid pressure, are adapted to engage and ride against the walls of the cylinder. The conventional ringless pistons have seen primary use in pumping abrasive materials, such as concrete to a specific location and have not seen extensive use in closed flow hydraulic systems.
U.S. Pat. No. 3,902,405 describes a ringless piston for use in a hydraulic system. The piston, as described in that patent, is formed of a material, such as polyurethane, and is provided with a running clearance with the cylinder wall, meaning that under unpressurized conditions, the lip of the seal will be spaced from the cylinder wall. However, it has been found that pistons utilizing materials, such as polyurethane, nylon, or other thermoplastic materials, cannot be satisfactorily operated at elevated temperatures above 250.degree. F. or 300.degree. F., as these polymers become gummy and tend to deteriorate.